In certain control systems, such as, occupancy, continuity or insulated joint detection in railroad track circuits, it is common practice to connect an a.c. transmitter at one point and to connect a receiver at another point. Thus, during the absence of a train, a broken rail or a bad insulated joint in the track circuit, the a.c. signals conveyed to the track by the transmitter are picked up by the receiver to signify a safe condition. When there is high electrical interference present in the tracks, such as, that caused by electrical propulsion currents, it is advisable from a security standpoint to take certain precautionary measures. For example, added security against a false and unsafe response of the receiver may be obtained by modulating the a.c. carrier of the transmitter in a simple ON-OFF fashion and by making the receiver only responsive to the correct modulating and carrier frequencies. Additionally, in the interest of safety and wiring considerations, it is advantageous to employ a receiver which is self-excited. That is, it is desirable to have the receiver derive its entire output power from the modulated carrier input signal without using any other external power supply, both to avoid high gain circuits which might be sensitive to noise, and on occasion, to simplify wiring. However, a signal-powered receiver must be efficient in operation in order to minimize the power requirements of the transmitter. The conventional approach of meeting the above requirements was very wasteful of the signal energy which resulted in the need of an excessively powerful transmitter. In practice, the picked up signals are fed to a carrier filter and, in turn, to a demodulator having a capacitor filter. It was necessary to provide a discharge resistor across the capacitor filter in order to allow the voltage to be pulled back down at the end of a modulation ON cycle. A coupling capacitor was interposed between the discharge resistor and a demodulation filter to prevent any demodulation d.c. from entering the modulation filter. It will be appreciated that the immediate discharge resistor not only dissipates an unusual and unnecessary amount of useful energy but also requires the use of the coupling capacitor for isolation purposes.